The electroplating is generally conducted by immersing an electrode and an object to be plated into a plating liquid containing a plating metal dissolved therein, and then applying an electrical current between both parties, the electrode as anode and the object to be plated as cathode. Here, when the object to be plated is a tube, plating on the inner surface side thereof becomes excessively insufficient compared to the outer surface side, since the inner surface is hidden from the electrode and the electrical current is therefore insufficient. On the other hand, as a countermeasure against the above, an auxiliary anode has been disposed in the tube so as to improve the electrical current distribution. In that case, when the tube is a straight tube, the auxiliary anode may be passed concentrically inside of its hollow as vertically immersing the straight tube, however, this method cannot be employed when the tube is a bent tube curved in the middle thereof.
Considering the foregoing, as a method for dealing with a case where the above-mentioned object to be plated is a bent tube curved in the middle thereof, there has been provided an auxiliary anode unit having an insulating spacer mounted to a flexible and linear auxiliary anode, which is inserted into the bent tube (for example, see Patent literature 1).    [Patent literature 1]: Japanese Patent Registration No. 3081558 (FIG. 9)